Heating assembly for an electric furnace with non-linear resistance heating elements



p 1968 E. 1.. KIRKMAN 3,

HEATING ASSEMBLY FOR AN ELECTRIC FURNACE WITH NON-LINEAR RESISTANCEHEATING ELEMENTS Filed Oct. 20, 1965 2 Sheets-Sheet l INVENTOR. J EARLL.KIRKMAN }L11N.UM

ATTORNEY Sept. 10, 1968 E. KIRKMAN 3,401,225

HEATING ASSEMBLY FOR AN ELECTRIC FURNACE WITH NON-LINEAR RESISTANCEHEATING ELEMENTS Filed Oct. 20, 1965 2 Sheets-Sheet 2 LINE - TOCONTROLLED INPUT SYSTEMS INVENTOR. EARL L KIRKMAN BY 9l1 ALJW ATTORNEYUnited States Patent 3,401,225 HEATING ASSEMBLY FOR AN ELECTRIC FURNACEWITH NON-LINEAR RESIST- ANCE HEATING ELEMENTS Earl L. Kirkmau, 933 OldZayante Road, Felton, Calif. 95018 Filed Oct. 20, 1965, Ser. No. 498,210Claims. (Cl. 13-24) ABSTRACT OF THE DISCLOSURE An electric furnace inwhich resistive heating elements have a non-uniform or non-linearresistance to compensate for gradual and exponential thermal and radiantheating losses, which are greater at the extremities of the furnace andwhich gradually decrease as the central portion of the furnace isapproached.

The present invention relates in general to electric furnaces, and moreparticularly to an electric furnace employing resistive elements forconverting electric power into thermal energy.

Conventional tubular electric furnaces of the diffusion type or the typeemployed in the manufacture of semiconductor or photoconductor devicesgenerally comprise three sections, namely: an entrance zone, center zoneand an exit zone. It has been found that the electric furnaces of thetype referred to have characteristic thermal and radiation losses. Inthe entrance zone, the greatest thermal and-radiation losses appear atthe end of the furnace. The minimum thermal and radiation losses appearat the center of the furnace. Thus, as the entrance zone progressestoward the center zone, there is a gradual decrease in radiation andthermal losses. Similarly, the exit zone has its greatest thermal andradiation losses at the end of the furnace. The minimum thermal andradiation losses appear at the center of the furnace. Hence, as the exitzone progresses toward the center zone, there is a gradual decrease inradiation and thermal losses. The thermal and radiation lossesthroughout the center zone is at a minimum and substantially constant.The thermal and radiation losses in the entrance and exit zones areexponential in characteristics along the axis of the tubular furnace asthe measurements are taken from each end of the furnace and thenprogressing centrally therefrom.

An object of the present invention is to provide an electric furnace inwhich the heating elements thereof are constructed to compensate for thegradual characteristic thermal and radiant losses of the furnace.

Another object of the present invention is to provide an electricfurnace in which the resistive heating elements are formed with agradually varying resistance to compensate for the gradualcharacteristic thermal and radiant losses of the furnace.

Another object of the present invention is to provide an electricfurnace in which resistive heating elements are constructed in anon-uniform or non-linear arrangement to compensate for the gradualcharacteristic thermal and radiant losses of the furnace.

Another object of the present invention is to provide an electricfurnace in which resistive heating elements are constructed in anon-uniform or non-linear arrangement to compensate for gradual andexponential thermal and radiant losses in the electric furnace which aregreater at the extremities of the furnace and gradually decrease as thecentral portion of the furnace is approached.

Other and further objects and advantages of the present invention willbe apparent to one skilled in the art 3,401,225 Patented Sept. 10, 1968from the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view of a tubular heating assembly for anelectric furnace embodying the present invention.

FIG. 1 is an end elevation view of the tubular heating assembly shown inFIG. lwith diagrammatical illustrated electrical circuits.

FIG. 3 is a diagrammatic illustration of the heat source elements of thepresent invention employed in connection With the tubular heatingassembly shown in FIGS. 1 and 2.

Illustrated in FIGS. 1 and 2 is a tubular heating assembly 10 embodyingthe present invention, which heating assembly 10 will be encased in aconventional manner to form an electric furnace. The tubular heatingassembly 10 comprises a tubular electrical insulating ceramic housing-11. Disposed within the housing 11 concentrically therewith and spacedradially inward therefrom are suitable electrical insulating ceramicinner sections 13.

The tubular heating assembly 10 may be divided into three zones, namely:entrance zone A, central zone B and exit zone C. Supported by the innerwall of the inner ceramic sections 13 are uniformly wound primary orinner heating elements 21-23. The primary heating elements are disposedwithin the entrance zone A, center zone B and exit zone C, respectively.Each primary or inner heating element is made of suitable resistancematerial uniformly wound in spiral or helical configurations along theinner wall of the inner sections 13 and the uniformly wound windingsthereof are suitably spaced apart and have uni-form resistancethroughout the turns thereof. The axis of the heating elements 21-23 arecoincident with the axis of the housing 11. The turns are convenientlyseparated and insulated by well-known ceramic T-form separators. It iswithin the contemplation of the present invention that the primaryheating elements be a continuous and single winding.

A suitable source of electric power, such as a control system 25,supplies electric power to the primary heating elements 21-23 forproducing furnace heat. Toward this end, transformers 26-28 (FIG. 3)have the primary windings thereof connected to the source of power 25and have the secondary windings thereof connected to the primary heatingelements 21-23, respectively. Should a more simplified control system bedesired, the heating elements may be connected directly to a source ofelectrical energy.

It has been found that the tubular heating assemblies of the typereferred to have characteristic thermal and radiation losses. In theentrance zone A, the greatest thermal and radiation loss appears at theend of the tubular housing 11 adjacent to the entrance opening 12. Theminimum thermal and radiation loss for the entrance zone A appears atthe end thereof adjacent to the center zone B or more accuratelyintermediate the ends of the center zone B. As the entrance zone Aprogresses from the end of the housing 11 adjacent to the entranceopening 12 toward the center zone B taken in the axial direction of thehousing 11, there is a gradual decrease of thermal and radiation lossesfor the assembly 10, which thermal and radiation losses for the pathdescribed, when projected in the form of a graph, appear as anexponential curve.

In a like manner, the exit zone C has its greatest thermal and radiationloss at the end of the tubular housing 11 adjacent to the rear openingin the housing 11. The minimum thermal and radiation loss for the zone Cappears at the end thereof adjacent to the center zone B or moreaccurately intermediate the ends of the center zone B. As the exit zoneC progresses from the end of the housing 11 adjacent to the exit openingin the housing 11, toward the center zone B taken in the axial directionof the tubular housing 11, there is a gradual decraese of thermalandradiation losses for the tubular heating assembly 10, which thermal andradiation losses for the path described, when projected in the form of agraph, appear as an exponential curve.

Across the center zone B of the tubular heating assembly 10, the thermaland radiation losses are relatively uniform and constant. It is thecenter zone B. that has the maximum temperature control and minimumthermal and radiation losses.

According to the present invention, secondary outer heating elements 30and 31 (FIG. 3) are mounted in the entrance zone A and the exit zone C,respectively, between the outer ceramic tubular insulating housing 11and the inner ceramic insulating sections 13 to compensate for theabove-mentioned thermal and radiation losses and extend partially withinthe center zone B. Since the radiation and thermal losses in theentrance zone Aand the exit zone C, respectively, are graphicallyrepresented in the form of an exponential curve, the secondary outerheating elements 30 and- 31 are non-uniform and nonlinear, respectively,to produce uniform heating in the axial direction throughout theentrance zone A and the exit zone C, respectively.

Each secondary or outer heating element is made of suitable resistancematerial Wound in spiral or helical configurations between the innerwall of the tubular housing 11 and the outer wall of the ceramicsections 13 and the turns of the windings thereof are suitably spacedapart. The axis of the heating elements 30 and 31 are coincident withthe axis of the housing 11. The turns are conveniently separated andinsulated by well-known ceramic T-formseparators. Thus, the primaryheating elements 21-23 are electrically insulated from the secondaryheating elements 30 and 31.

A suitable source of electric power, such as a control system 40,supplies electric power to the secondary heating elements 30 and 31 forproducing furnace heat. Manually cont-rolled rheostats 41 and 42 may beprovided to control the current flow through the secondary heatingelements 30 and 31, respectively. It is within the contemplation of thepresent invention to employ automatic controls in lieu of the manualcontrols 41 and 42 for fine resistance adjustment and fine currentregulation. The opposite ends of the heating elements 30 and 31 may beconnected to opposite end terminals of the primary winding and a sourceof power may be connected to adjacent inner taps of the secondarywindings. In this manner, the secondary and primary windings may beconnected in series and, yet, electrically insulated from one another.There is present minimum thermal insulation between the primary andsecondary windings.

In order to compensate for the exponential rise in the thermal andradiation losses in the entrance zone A and the exit zone C,respectively, the secondary heating elements 30 and 31 are,respectively, non-uniform and nonlinear in resistance taken on aturn-'by-turn basis and progressing in the axial direction of thehelical or spiral windings thereof.

The present invention provides for a varying resistance for thesecondary heating elements 30 and 31 on a turnby-turn basis tocompensate for the exponentially rising thermal and radiation losses bycreating through the secondary heating elements 30 and 31 acomplementaryheating curve along the axis of the housing 11 in the entrance zone Aand the exit zone C, so that the heat produced by the secondary windings30 and 31 along the referred to axis substantially equals the referredto thermal and radiant losses along the referred to axis. In thismanner, the heating temperature along the axis of the housing 11 withinthe entrance zone A and the exit zone C is maintained uniform.

Toward this end, the preferred embodiment provides for the securing ofheating element material, such as the material from which the secondaryheating elements 30 and 31 are made to each turn individually of thesecondary heating elements 30 and"31, thereby decreasing the resistanceof the turns of the secondary heating elements 30 and 31. The quantityof additional material applied to the respective turns of the heatingelements 30 and 31, respectively, will be non-uniform and non-linear andwill vary from turn-to-turn. For example, with regard to the entrancezone A, thelminimum added quantity of heating element material will beadded to the turn contiguous to the entrance opening 12. On. the otherhand, the maximum added quantity of heating element material will beadded to the turn contiguous with or within the center zone B. Thequantity of material to be added to each turn of the secondary heatingelement 30 between the aforementioned minimum and maximum quantitieswill gradually'and successively vvary between the aforementionedextremes. I 1

In a similar manner, the minimum quantity of additional material appliedto the secondary heating element 31 will be on the turn thereof in exitzone C contiguous to the exit opening of the housing 11. On the otherhand, the maximum added quantity of heating element material will beadded to the turn contiguous with or within the center zone B. Thequantity of material to be added to each turn of the secondary heatingelement 31 between the aforementioned minimum and maximum quantitieswill gradually and successively vary between the aforementionedextremes.

It is recognized that the secondary heating elements 30 and 31 can beintegrally formed to achieve the intended results without the procedureof adding heating element material. Stated otherwise, initially and atthe time of manufacture, the heating elements 30 and 31 can be formedwith non-uniform and non-linear resistance varying at a successive andincreased rate. Also, it is apparent that an entire turn need not bechanged, but a portion thereof could have an added quantity of heatingelement material to effect the above-described intended results. Lastly,it is within the contemplation of the present invention that distancesbetween turns of windings can be varied to achieve the non-uniformheating to compensate for the exponential thermal and radiant heatinglosses in the entrance zone A and the exit zone C.

From the foregoing, it is to be observed that the present inventionachieves in a cylindrical or tubular furnace substantially a fiatthermal response along the axis of the furnace throughout the axiallength thereof.

While helically wound heating resistance elements have been referred to,it is apparent that the heating elements may constitute tungsten wiremesh, sheet elements, ceramic heating elements or any other suitableresistance heating elements.

It is to be understood that modifications and variations of theembodiment of the invention disclosed herein may be resorted to withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

Having thus. described my invention, what I claim as newand desire toprotect by Letters Patent is:

1. A heating assembly for an electric furnace comprising a housing, atubular wall in said housing constituting a heating chamber, anon-linear resistance in said housing for heating said chamber and forrendering the heat uniform in saidchamber along the axis of the entirelength of said tubular wall, and means for conducting electric energy tosaid resistance.

2. A heating assembly for an electric furnace comprising a housing, atubular wall in said housing constituting a heating chamber, annon-uniform resistance in said housing for heating said chamber and forrendering the heat uniform in said chamber along the axis of the entirelength of said tubular wall, and means for conducting electric energy tosaid resistance.

3. A heating assembly for an electric furnance with non-uniform heatloss characteristics comprising a housing, a wall in said housing forforming a heating chamber,

a non-linear resistance in said housing for heating said chamber tocompensate for the non-uniform heat losses for rendering the heat insaid chamber more uniform, and means for conducting electric energy tosaid resistance.

4. A heating assembly for an electric furnace with non-uniform heat losscharacteristics comprising a housing, a wall in said housing for forminga heating chamber, a non-uniform resistance in said housing for heatingsaid chamber to compensate for the non-uniform heat losses forrenderingthe heat in said chamber more uniform, and means for conducting electricenergy to said resistance.

5. A heating assembly for an electric furnace with non-uniform heat losscharacteristics comprising a housing, a wall in said housing for forminga tubular heating chamber, said heat loss characteristics beingnon-uniform along the axis of said chamber, a non-linear resistancedisposed in said housing along an axis coincident with the axis of saidchamber to compensate for the nonuniform heat losses along said axis ofsaid chamber for rendering the heat more uniform in said chamber alongthe axis thereof, and means for conducting electric energy to saidresistance.

'6. A heating assembly for an electric furnace with nonuniform heat losscharacteristics comprising a housing, a wall in said housing for forminga tubular heating chamber, said heat loss characteristics beingnon-uniform along the axis of said chamber, a non-uniform resistancedisposed in said housing along an axis coincident with the axis of saidchamber to compensate for the non-uniform heat losses along said axis ofsaid chamber for rendering the heat more uniform in said chamber alongthe axis thereof, and means for conducting electric energy to saidresistance.

7. A heating assembly for an electric furnace with exponential heat losscharacteristics comprising a housing, a wall in said housing for forminga tubular heating chamber, said heat loss being exponent-i al along theaxis of said chamber, a non-linear resistance disposed in said housingalong an axis coincident with the axis of said chamber to compensate forthe exponential heat losses along said axis of said chamber forrendering the heat more uniform in said chamber along the axis thereof,and means for conducting electric energy to said resistance.

8. A heating assembly for an electric furnace with exponential heat losscharacteristics comprising a housing, a wall in said housing for forminga tubular heating chamber, said heat loss being exponential along theaxis of said chamber, an non-uniform resistance disposed in said housingalong an axis coincident with the axis of said chamber to compensate forthe exponential heat losses along said axis of said chamber forrendering the heat more uniform in said chamber along the axis thereof,and means for conducting electric energy to said resistance.

9. A heating assembly for an electric furnace with exponential heat losscharacteristics comprising a housing, a wall in said housing for forminga tubular heating chamber, said heat loss characteristics beingexponential along the axis of said chamber, a heating resistancedisposed in said housing along an axis coincident with the axis of saidchamber, said heating resistance being formed with its resistance variedas said heating resistance progresses axially to compensate for theexponential heat losses along said axis of said chamber for renderingthe heat more uniform in said chamber along the axis thereof, and meansfor conducting electric energy to said heating resistance.

10. A heating assembly for an electric furnace with exponential heatloss characteristics comprising a housing, a wall in said housing forforming a tubular heating chamber, said heat loss characteristic beingexponential along the axis of said chamber, a primary uniform heatingresistance disposed in said housing along an axis coincident with theaxis of said chamber for heating said chamher, a secondary heatingresistance disposed in said housing along an axis coincident with theaxis of said chamber, said secondary heating resistance being formedwith its resistance varied as said secondary heating resistanceprogresses axially to compensate for the exponential heat losses alongsaid axis of said chamber for rendering the heat in said chamber moreuniform along the axis thereof, and means for conducting electric energyto said heating resistance.

References Cited UNITED STATES PATENTS 2,768,277 10/1956 Buck et al.13-20 XR 884,230 4/1908 Spivak 219-407 XR OTHER REFERENCES Wong:Variable Constant-Gradient Furnace, IBM Technical Disclosure Bulletin,vol. 4, No. 7, Dec. 1961, page 39 (Copy in Scientific Library), 13-20.

ROBERT K. SCHAEFER, Primary Examiner.

M. GINSBURG, Assistant Examiner.

